CHEMORECEPTION 153 



suggests that weak physical forces are involved, and evidence has been 

 obtained that the salt-combining sites are anionic and strongly acidic. 

 As a consequence, the cation of a salt largely dominates stimulation. 



The Water Receptor 



The water receptor discovered by Evansand Mellon (1962 a) is the only 

 cell in the chemoreceptive hair of P/wrmia that is activated by water. 

 The most effective stimulus tested thus far is pure water. Aqueous 

 solutions of sucrose appear to depress the activity of the cell as a linear 

 function of the logarithm of osmotic pressure. At 5M sucrose the 

 frequency of impulses decreases to less than half of the maximum. 

 Other non-electrolytes (e.g., glycerol and mannose) also inhibit, but 

 no simple relation was found between parameters of the solution and 

 the degree of inhibition. Inorganic electrolytes also inhibit water res- 

 ponse, but in a more specific manner. Aqueous solutions of NaCl 

 begin to inhibit at 0-1 M. Inhibition appears to be complete at about 

 0-3M. Calcium chloride solutions inhibit at about 0-01 M. The 

 inhibition of water response is unrelated to spike activity in other 

 fibres. 



Modalities 



For the blowfly it is clear that water, sugars, protein, and unacceptable 

 compounds can be distinguished as different. To what extent these 

 four taste categories can be subdivided into other taste modalities is 

 not clear. Although there is evidence that different sugars act upon 

 different sites at the molecular level, they none the less stimulate the 

 same receptor. In the case of unacceptable compounds, that is, salts, 

 acids, alkaloids, alcohols, etc., it is probable that different sensory in- 

 put results from stimulation by the different compounds. If this 

 suggestion is confirmed it provides a peripheral mechanism for dis- 

 criminating among a number of compounds and indicates that the 

 modality 'unacceptable' is not a homogeneous one. 



Aside from incomplete electrophysiological data obtained with 

 Phormia, evidence for taste modalities has been derived principally 

 from the following sources: (1) observations of the behaviour of indi- 

 viduals conditioned to respond to a given chemical ; (2) determinations 

 of the additive or non-additive capacity of diverse stimuli ; (3) measure- 

 ment of the amount of sucrose drunk by honeybees after contamina- 

 tion with subliminal concentrations of different chemicals ; (4) com- 

 parison of the threshold changes for different compounds during 

 starvation; (5) localization of specific kinds of receptors. Much of the 



